1. Field of the Invention
The present invention relates to an imaging device, and more particularly to an infrared imaging device for use in temperature measurements, etc.
2. Description of the Related Art
Infrared sensors convert an infrared radiation from the surface of a subject into an electric signal and displays the image of the subject based on the electric signal for measuring a temperature distribution, etc. of the surface of the subject. The infrared sensors are required to have excellent temperature resolution so as to be able to distinguish small temperature differences and also to have a wide dynamic range so as to cover a wide range of temperatures that can be measured.
FIG. 1 of the accompanying drawings shows a conventional image sensor which includes at least bolometers 1 thermally isolated from a substrate (not shown) and serving as respective pixels, a vertical shift register 9, a horizontal shift register 11, and integrating circuits 14, 15 for converting a change in resistance into a change in voltage. When infrared energy is radiated from a subject to be imaged, the radiated infrared energy is converged onto the imaging device by an optical system, causing changes in the temperature of the bolometers 1, which result in changes in the resistance of the bolometers 1. The pixels are successively selected by the vertical shift register 9 and the horizontal shift register 11, and the changes in the resistance of the bolometers 1 are successively converted into electric signals by the integrating circuits 14, 15. For further details, reference should be made to Japanese laid-open patent publication 105794/1996.
Japanese laid-open patent publication 1989-102330 discloses another conventional imaging device which comprises an infrared sensor, a subtractor, a central temperature setting circuit, a gain selector, and a peak hold circuit. A temperature distribution of a subject to be imaged is converted into an electric signal by the infrared sensor. Signals from the pixels are successively read as with ordinary television signals. The peak hold circuit holds maximum and minimum levels of the pixel signals that have been read, and the gain of the gain selector is controlled to bring the signal levels into a desired dynamic range. The user of the imaging device manually sets the central temperature setting circuit to a desired central temperature.
A thermal infrared imaging device revealed in Japanese laid-open patent publication 1996-46870 has an automatic level controller and an automatic gain controller. The automatic level controller controls an offset level to be added to a video signal such that the signal levels of half of total pixels will be in a lower half of the dynamic range and the signal levels of the remaining half of total pixels will be in an upper half of the dynamic range. The automatic gain controller controls a gain such that the signal levels of the pixels will be in 10% to 90% of the dynamic range.
Japanese laid-open patent publication 137062/1995 shows an imaging device in which the difference between bright and dark levels of captured images is increased by an automatic gain control (AGC) process.
An infrared imaging device disclosed in Japanese laid-open patent publication 107074/1990 has a gain correcting circuit for correcting sensitivity variations of pixels of the infrared imaging device to suppress fixed pattern noise (fixed DC level variations between the pixels) which is generated when the temperature of a subject being imaged is greatly changed.
The conventional imaging devices referred to above, however, suffer the following shortcomings:
(1) In the image sensor disclosed in Japanese laid-open patent publication 105794/1996, any change in the temperature of the bolometers due to incident infrared radiation is very small, and the image sensor itself has a very large dynamic range. For example, when a subject having a temperature difference of 1.degree. C. is imaged, the temperature of the bolometers changes only by 2.times.10.sup.-3 .degree. C. Even when a subject having a temperature of several hundred degrees centigrade is imaged, any change in the temperature of the bolometers does not exceed 1.degree. C. Therefore, the dynamic range of the thermal infrared imaging device is determined by the dynamic range of an amplifier, etc. which amplifies a video signal from the image sensor.
For imaging a subject having a low-illuminance area and a high-illuminance area, since the amplification factor of the amplifier is lowered so that signal levels from the low-illuminance area and the high-illuminance area will enter the dynamic range, the temperature resolution is lowered, resulting in a failure to observe a small illuminance distribution on the surfaces of the subjects.
(2) The imaging devices disclosed in Japanese laid-open patent publications 102330/1980, 46870/1996, and 137062/1995 have the same problems as described above because they have AGC circuits or similar circuits and the amplification factor of the amplifier and the offset level for the video signal are controlled by the AGC circuits or similar circuits such that the signal levels of an overall subject to be imaged will enter the dynamic range.
The infrared imaging device disclosed in Japanese laid-open patent publication 107074/1990 fails to solve the above problems because it corrects the gain variations of individual pixels for producing uniform images.